1. Field of the Invention
The present invention relates to an apparatus for compensating for a black level of an image sensor.
2. Description of the Related Art
An image sensor, a semiconductor element for converting an optical image into an electrical signal, is widely employed for digital cameras, mobile communications terminals, and the like. The image sensor includes pixel arrays, and the like, which detect light generated from a subject and convert the detected light into an electrical value.
Recently, as an image sensor has been implemented to have a higher amount of pixels and a smaller size, pixel size has been reduced and a fabrication process has become finer, resulting in an increase in the influence of a dark current, dark current noise generated by the dark current, photon shot noise, MOS device noise, and the like. The increase in the dark current brings about problems such as the degradation of device performance and of charge storage capability. In particular, the increase in the dark current causes a serious problem such as degradation of the performance of the image sensor and of charge storage capability in a low intensity illumination environment.
Thus, in an effort to solve the problem of the dark current, currently, a black level value caused by the dark current is compensated for by using a black level compensation circuit.
In order to extract only a dark current value generated by heat, irrespective of an optical image, the black level compensation circuit extracts a black level value by using a pixel data value (namely, a black pixel value) of an optical black area completely blocked against light.
However, as the pixel size is reduced and the fabrication process becomes finer, dark current noise, photon shot noise, MOS device noise, and the like, are relatively increased as compared with a practical signal value. Thus, when the black level compensation circuit is in use, a variation of an inter-frame black level value is increased at a low intensity of illumination, causing a phenomenon in which a device screen flickers. In spite of an identical environment and an identical pixel, an output value of a pixel varies over time, due to the dark current noise, photon shot noise, MOS device noise, and the like, causing deviation between frames, and the inter-frame deviation is further increased at a low intensity of illumination in which a great deal of analog gains are used, to cause screen flicker.
That is, the difference between the pixels of frames causes a difference between black level values of the frames, and as a result, a black level value having a large deviation is subtracted from each frame in the same environment, causing the screen flickering phenomenon.
In order to reduce the screen flickering phenomenon, the number of pixels used to obtain a black level value may be increased to reduce the change in the black level value, which, however, inevitably increases the size of the image sensor, so this method is not practically desirable.
Another method of obtaining a black level value by using black pixel values during a few frames to tens of frames, rather than increasing the black pixels to obtain a black level, to reduce a change in the black level value to thereby minimize the screen flickering phenomenon has been proposed, but this method has a problem in that a convergence speed of the black level value is low.
Namely, the black level value sharply changes when an analog gain changes, but when an actually applied black level value is obtained by using black pixel values of a few frames to tens of frames as mentioned above, the black level value is gradually converged into a value fitting a current environment, causing a problem such as an AE (Auto-Exposure) speed, and the like.